Unexpected vertical black patterns or artifacts such as lines, dots, dashes, etc., have been a common image artifact in binary document images produced by production paper scanners. The vertical black patterns are not part of the original document image, however, the patterns are the result of pieces of dust or dirt blocking light between an illumination light source and the paper documents being scanned. For example, pieces of dust or dirt which come to rest on transparent image guides in an illumination path of the scanner will result in shaded lines upon the document moving through the scanner. When the document is electronically captured by a CCD device, the shaded lines will appear in the digital document image. The shaded vertical lines within a grey scale image will be indistinguishably treated as part of image content. After an adaptive thresholding is applied to the grey scale image, the shaded lines will show up as black lines in the binary image. A method which removes unwanted vertically disposed artifacts is described in the above-mentioned co-pending application Docket No. 78434.
When a paper document is scanned by a production scanner, the resulting digital image is often skewed. The causes of a skewed image are either that the document is not fed properly, or the application of uneven paper carrying forces in a scanning system while transporting a paper document. The skew of a document image produces an unpleasant effect on viewing an image, hence an image skew correction is becoming a desired feature in a document scanning system (see, for example, U.S. Ser. No. 09/006,565.
If the phenomenon of vertical line artifacts occur in a skew image or document 1 as shown in FIG. 1A, a vertical line artifact 3 will turn into a diagonal line artifact 5 in the skew corrected image or document 1' as shown in FIG. 1B.